SF part IV: What will I be?

If you stumbled upon this post, you are forgiven to find the next phrases a little quaint. That is because it took three posts (part I, part II and part III) to come to the point where the structural framework suddenly realises that it is still very little. It therefore starts asking: what can I be when I grow up? And like any nice person, you answer: anything you like, sweet little thing.

And like always when we are trying to be nice, we lie. We
say ‘anything’, but really mean ‘useful, reliable, and easy going’. Hopefully
the structural framework is going to be ok with that, because GeoConnect³d is a
parent that already has planned its child’s career.

So far on our wish list we find two important elements. The
framework needs to exist of structures that are related, but preferably not in
a complicated way that reminds us too much of structural geology, or
mathematics. Also, we want it to look like a collection of geological units. Sounds
simple, right? Well, anything too straightforward is to be regarded as
suspicious.

Geological units are big chunks of the Earth’s crust, and
therefore quite different from the faults that we mostly have been discussing
as the building blocks of our framework. Try to imagine what is left after
leaving 1 trillion cubic kilometres of molten rock spinning round for 4.5
billion years: no way that is going to end up as something nice and organised. It
is not a coincidence that geologists prefer hammering rocks instead of looking
at them, and seem to have an urge to get dirty whenever they go outside. You
become what you study, you become messy.

Now imagine that you are looking at geological units on a
map. You may think that it is similar to a stone wall, a collection of more and
less interesting rocks with some cement in between to keep them together. Good
thinking, but wrong. Examples of geological units are a terrane, a fault block,
or a basin. These look like different entities because they acted as separate units
during some part of geological history.

That may sound as a satisfying definition of a geological unit, but the most important question
is: why are they different from neighbouring units? And why was the geological
history different from one unit to the other? The reason is that units have one
or more limits, and it is hard to
overestimate the importance of limits.

Those limits can obviously be faults. A good example is a
graben, which is by definition a fault bounded area of subsidence. If fault
activity is reversed, it becomes a horst, a fault bounded area of uplift. In
these examples, the geological units graben and horst are blocks that are being
defined by the faults, or the structural framework. Defined should be taken
very literally here: the graben exists only if its bounding faults exist and
are active.

But faults are just some of the structures of interest. A
terrane, for example, is a piece of continental crust that was once separated,
all by itself, surrounded by nothing but ocean. Now it is bounded by suture
zones, the lines where the oceanic crust was subducted before the terranes
crashed into each other. Well, very slowly crashed into each other, we are
talking geology. Such suture zones are not really faults, but they certainly
are structures.

And then there are the sedimentary basins. Do these have
limits? Well, they do if you think in 3D. A basin is typically defined by a
basal unconformity, marking the start of subsidence and sedimentation cycles
that are typical for a certain basin.

So, we are calling unconformities structures now? Yes, but
only if they define geological units. The goal of a structural framework is to
structure things, to tie them together. This means that anything that does this
will be treated as a structure in our framework.

So far so good, but this is only the beginning. We have a clearer idea of what the structure framework will be: a network of limits and the different geological units delimited by them (see the figure below). Now it is time to start combining the geological information of our areas of interest – covering limits and units from pan-European to local scales – into one coherent model. Bringing up a structural framework is a tiring task, and it isn’t even going through puberty yet.

Structural framework: example of how to go from theory to practice. A) The terrane map of Europe (Jähne, 2014) that we discussed last week. B) Some large-scale limits in Europe, simplified from the previous map. C) Some large-scale units (coloured areas) can then be defined based on their limits. The more limits you trace, the more units you find. Click here to enlarge the figure.

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